Next you can head west of Violet City and run into a cave entrance. Inside will be a puzzle that you have to put together.
Once you discover it was Kabuto, the floor will drop out and down you will fall into the cave below. (If nothing happens, you put the puzzle together wrong) Inside are a bunch of Pokémon called Unown. They have no gender, but they do have many different shapes. Don't get confused though, they all have the same Pokédex entry.
To unlock more shapes you will have to surf, strength, break rocks, towards the 3 other entrances to the cave (Like the Kabuto one where there is a puzzle to solve.) You can find them by surfing on the Lower Level of Union Cave. You need Strength to get two of them.After you solve a puzzle go into the cave and catch at least 3 different Unown shapes. It doesnt Matter if they were sent to the Computer
Exit the cave and a Scientist will upgrade your Pokédex for you. Now after you catch a Unown shape it will be recorded under here and you can take a look to see which ones you have by pressing Select while in the Pokédex and then choosing the last option.
After collecting all 26 Unown, talk to the scientist again and he will let you print out your Unown shapes.
When looking in this part of the Pokédex you will notice that every Unown make up a different letter of the English Alphabet. And in the box below the picture it has an English word made up of Unown letters. Go to the page to find out what they mean by clicking here

In Crystal

Each of the four puzzles hold a secret. By discovering the secret of the word written on the back room, one can open a secret passage to a room with four items and a second room with a special message about the Unowns.
When The Keyword in one of the rooms is Flash. Use "FLASH". When You do a door will open and you will enter a room With 4 Items In. The items are:
Top-Left: Heal Powder
Top-Right: Energy Root
Bottom-Left: Gold Berry
Bottom-Right: Moon Stone
When The Keyword in another one of the room is Water. This means "water". Have a Pokémon hold a Water Stone. When You do a door will open and you will enter a room With 4 Items In. The items are:
Top-Left: Stardust
Top-Right: Star Piece
Bottom-Left: Mysteryberry
Bottom-Right: Mystic Water
When The keyword in another one of the rooms is Escape. Use an ESCAPE ROPE. Upon re-entering the room, you'll notice the door opening. You Will Enter Another Room With 4 Items In. They Are:
The items are:
Top-Left: Heal Powder
Top-Right: Energy Powder
Bottom-Left: Berry
Bottom-Right: Psncureberry
The Keyword in the final room is HO-OH. Place Ho-oh at the head of your party, and the door will open. Inside the room...again you will find another 4 items. These are:
Top-Left: Revival Herb
Top-Right: Charcoal
Bottom-Left: Gold Berry
Bottom-Right: Mysteryberry